Spontaneous unit activity in the globus pallidus following cumulative injections of morphine in phenobarbital- or chloral hydrate-anesthetized rats

The effect of systemically administered morphine on spontaneous unit activity, recorded from the globus pallidus, was evaluated in this study. A wide range of doses of morphine was administered in order to characterize the influences of morphine on pallidal activity in a dose-related manner. Two separate studies were conducted. In the first study, a semichronic preparation, lightly anesthetized with phenobarbitol, was used. Acutely prepared, chloral hydrate-anesthetized rats were utilized in the second experiment. With phenobarbital, morphine caused a significant reduction in pallidal activity in 75% of the cells recorded, whereas only 45% of the pallidal cells responded similarly to injection of morphine in chloral hydrate-anesthetized animals. However, the dose of morphine required to decrease unit activity was substantially less with chloral hydrate than with phenobarbital anesthesia.